Advances in the biological, biomedical and pharmaceutical sciences have accelerated the pace of research and diagnostics to a level unparalleled to the past. With sequences of whole genome becoming available quickly and successively, the assembly of large libraries of small molecules, the ability to move pharmaceutical development, clinical diagnostic tests and basic research from a reductionist to a whole system approach quickly all demand assays that facilitate high throughput analyses. Molecules no longer need to be singly analyzed for their effects on a lone process. Instead, the effects of many molecules on several biological systems can be studied simultaneously if appropriate, fast, reliable, and accurate assays are available.
Efficient, reliable and accurate assays for determining the occurrence of a certain biospecific events, e.g., enzyme inhibition in a cell-free environment or for assessing cell viability can be used to rapidly discover potential new pharmaceutical agents and to determine the cytotoxic effect or cell proliferation effect of such agents on cells. For instance, cancer pharmaceutical research often endeavors to identify compounds that selectively kill rapidly dividing cells, a primary characteristic of cancer cells. High throughput screens of compound libraries, coupled with efficient cell viability assays, can swiftly identify such compounds as potential cancer drugs. The efficacy of a candidate compound on cell viability can be assayed by detecting ATP since ATP production is realized only in metabolically active live cells; residual ATP is rapidly degraded upon necrotic cell death. See U.S. patent application Ser. No. 09/813,279, filed Mar. 19, 2001, entitled “Improved Method for detection of ATP” (assignee: Promega) which is incorporated by reference in its entirety. In another example, the identification of potential drug compounds that should move forward in the drug development process can be made by determining the effects of these compounds on cytochrome P-450 enzyme activity. See U.S. application Ser. No. 10/665,314, filed Sep. 19, 2003, entitled “Luminescence-based methods and probes for measuring cytochrome P-450 activity” (assignee: Promega) which is incorporated by reference in its entirety. In a final example, proteases represent a large and important group of enzymes involved in diverse physiological processes including blood coagulation, inflammation, reproduction, fibrinolysis, and the immune response. The identification of protease inhibitors may be useful for the investigation, treatment or management of disease states caused by or characterized by the alteration in the activity of specific proteases. See U.S. Provisional application No. 60/353,158, filed Feb. 1, 2002 entitled “Bioluminescent Protease Assay” (assignee: Promega), which is incorporated by reference in its entirety. Assay systems like these not only facilitate the evaluation of a substance on cell viability or proliferation in a cellular environment or on the occurrence of a biospecific reaction, but also permit high throughput screens that can rapidly test thousands of compounds, streamlining new drug discovery.
The use of reporter molecules or labels to qualitatively or quantitatively monitor molecular events is well established in assays used for medical diagnosis, for the detection of toxins and other substances in industrial environments and for basic and applied research in biology, biomedicine and biochemistry. Reporter molecules or labels in such assay systems have included radioactive isotopes, fluorescent agents, enzymes, including light-generating enzymes such as luciferase. Desirable characteristics of any reporter molecule systems include safe, quick and reliable application and detection. Luminescent systems are among the most desirable since they are exceptionally safe and sensitive.
Light-emitting systems have been known and isolated from many luminescent organisms, including certain bacteria, protozoa, coelenterates, mollusks, fish, millipedes, flies, fungi, worms, crustaceans, and beetles. Those enzymes isolated from beetles, particularly the fireflies of the genera Photinus, Photuris and Luciola and click beetles of genus Pyrophorus have found widespread use in reporter systems. In many of these organisms, enzymatically catalyzed oxidoreductions take place in which the free energy change is utilized to excite a molecule to a high-energy state. When the excited molecule spontaneously returns to the ground state, visible light is emitted. This emitted light is called “bioluminescence” or “luminescence”. Luminescent luciferase-based assays have been developed to monitor or measure kinase activity, P-450 activity, and protease activity. See, for instance, U.S. application Ser. No. 10/665,314, filed Sep. 19, 2003 (P-450 activity); U.S. patent application Ser. No. 09/813,279, filed Mar. 19, 2001 (kinase activity); and U.S. provisional application no. 60,353,158, filed Feb. 1, 2002 (protease activity), commonly owned by Promega Corporation.
Genetic reporter systems are widely used to study eukaryotic gene expression and cellular physiology. Applications include the study of receptor activity, transcription factors, intracellular signaling, mRNA processing and protein folding. Currently, luciferase genes from a wide variety of vastly different species, particularly the luciferase genes of Photinus pyralis (the common firefly of North America), Pyrophorus plagiophthalamus (the Jamaican click beetle), Renilla reniformis (the sea pansy), and several bacteria (e.g., Xenorhabdus luminescens and Vibrio spp), are extremely popular luminescence reporter genes. Reference is made to Bronstein, et al. (1994) Anal. Biochem., Vol. 219, pp. 73-82, for a review of luminescence reporter gene assays. Firefly luciferase is also a popular reporter for ATP concentrations, and in that role is widely used to detect biomass. Various other reporter applications of luciferases have been described in the scientific literature. Luminescence may be produced by other enzymes when mixed with certain synthetic substrates; such as alkaline phosphatase mixed with adamantyl dioxetanes, or horseradish peroxidase mixed with luminol.
Luciferase genes are widely used as genetic reporters due to the non-radioactive nature, sensitivity, and extreme linear range of luminescence assays. For instance, as few as 10−20 moles of the firefly luciferase can be detected. Consequently, luciferase assays of gene activity are used in virtually every experimental biological system, including both prokaryotic and eukaryotic cell cultures, transgenic plants and animals, and cell-free expression systems. Similarly, luciferase assays of ATP are highly sensitive, enabling detection to below 10−16 moles of ATP.
Luciferases generate light via the oxidation of enzyme-specific substrates, called luciferins. For firefly luciferase and all other beetle luciferases, this is done in the presence of magnesium ions, oxygen, and ATP. For anthozoan luciferases, including Renilla luciferase, only oxygen is required along with the luciferin. Generally, in luminescence assays of genetic activity, reaction substrates and other luminescence-activating reagents are introduced into a biological system suspected of expressing a reporter enzyme. Resultant luminescence, if any, is then measured using a luminometer or any suitable radiant energy-measuring device. The assay is very rapid and sensitive, and provides gene expression data quickly and easily, without the need for radioactive reagents. Reporter assays other than for genetic activity are performed analogously.
The conventional assay of genetic activity using firefly luciferase has been further improved by including coenzyme A (CoA) in the assay reagent to yield greater enzyme turnover and thus greater luminescence intensity. (Promega Luciferase Assay Reagent, Cat. No. E1500, Promega Corporation, Madison, Wis.; see U.S. Pat. No. 5,283,179, issued Feb. 1, 1994.) Using this reagent, luciferase activity can be readily measured in luminometers or scintillation counters. The luciferase reaction, modified by the addition of CoA to produce persistent light emission, provides an extremely sensitive and rapid assay for quantifying luciferase expression in genetically altered cells or tissues.
Dual reporters are commonly used to improve experimental accuracy. The term “dual reporter” refers to the simultaneous expression and measurement of two individual reporter enzymes within a single system. In genetic reporting, examples that currently benefit from dual-reporter assays include individual cells or cell populations (such as cells dispersed in culture, segregated tissues, or whole animals) genetically manipulated to simultaneously express two different reporter genes. Most frequently, the activity of one gene reports the impact of the specific experimental conditions, while the activity of the second reporter gene provides an internal control by which all sets of experimental values can be normalized. Normalizing the activity of the experimental reporter to the activity of the internal control minimizes experimental variability caused by differences in cell viability or transfection efficiency. Other sources of variability, such as differences in pipetting volumes, cell lysis efficiency and assay efficiency, can be effectively eliminated. Thus, dual reporter assays often allow more reliable interpretation of the experimental data by reducing extraneous influences.
In genetic reporting, examples that currently benefit from dual-reporter assays include individual cells or cell populations (such as cells dispersed in culture, segregated tissues, or whole animals) genetically manipulated to simultaneously express two different reporter genes. Most frequently, the activity of one gene reports the impact of the specific experimental conditions, while the activity of the second reporter gene provides an internal control by which all sets of experimental values can be normalized.
Cell-free reconstituted systems that may benefit from dual-enzyme reporter technology are cellular lysates derived for the simultaneous translation, or coupled transcription and translation, of independent genetic materials encoding experimental and control reporter enzymes. Immuno-assays may, likewise, be designed for dual-reporting of both experimental and control values from within a single sample.
Currently, genes encoding firefly luciferase (luc), chloramphenicol acetyl transferase (CAT), beta-galactosidase (lacZ), beta-glucuronidase (GUS) and various phosphatases such as secreted alkaline phosphatase (SEAP) and uteroferrin (Uf; an acid phosphatase) have been combined and used as co-reporters of genetic activity. The following references provide representative examples of these various reporter genes used in combined form for the purpose of dual-reporting of genetic activity: luc and GUS: Leckie, F., et al., 1994; luc and CAT, and luc and lacZ: Jain, V. K. and Magrath, I. T., 1992; CAT and lacZ: Flanagan, W. M. et al., 1991; SEAP and Uf: Kondepudi, et al., 1994. See also Promega Dual-Luciferase® Reporter Assay system as well as Promega pGL3 Luciferase Reporter Vectors (available from Promega Corporation, Madison, Wis.) as well as U.S. Pat. Nos. 5,744,320 and 5,670,356 (assignee: Promega Corporation), which are incorporated by reference in their entirety.
When luciferase is combined with a sample for the purpose of detecting a product such as ATP or the occurrence of a biospecific event, e.g., inhibition or activation of caspase or P-450 activity, either in an enzyme assay or single/dual reporter assay format, one or more of the compounds in a chemical library used for high throughput drug screening may adversely interact with luciferase and thus interfere with the assay. For instance, in a caspase assay compounds that only inhibit caspase will result in decreased luminescence and would not be easily distinguishable from compounds that only inhibit luciferase activity which also decreases luminescence. There is a need for luciferase-based assays with improved tolerance for compound interference, especially when employed in high throughput screening procedures.